Rechargeable cells or batteries are electrochemical energy storage devices for storing and retaining an electrical charge and later delivering that charge as useful power. Familiar examples of rechargeable electrical storage cells are lithium-ion (Li-on) cells and nickel-cadmium cells used in various portable devices and lead-acid cells used in automobiles. Another type of electrical storage cell is the nickel oxide/pressurized hydrogen electrical storage cell, commonly called the nickel-hydrogen electrical storage cell, which is used in spacecraft applications.
Although electrical storage cells are designed for excellent reliability, there is always the chance of a failure. One common failure is when a cell, in a battery comprised of an array of series-connected cells, is defective and, therefore, has diminished storage capacity. When the battery discharges, the defective cell fully discharges prior to the other non-defective cells in the array. As the battery remains in discharge mode, the defective cell becomes overdischarged resulting in a reversal of the defective cell's voltage. This adversely affects the battery's operation because the defective cell is no longer able to carry the same current load as the other cells in the array. Another common failure is when there is an open-circuit failure, in which there is no longer a conducting path through the cell. In the event of such open-circuit failure of a single cell in a series-connected array of cells, all of the storage capacity of the array is lost. In a spacecraft battery, for example, a loss of the battery's storage capacity can lead to failure of the mission.
A bypass around a potentially defective or failed cell is required to prevent loss of the storage capacity of the entire array. The bypass must not conduct when the electrical storage cell is functioning properly, but it must activate to provide an electrically conductive bypass when the electrical storage cell fails. An example circuit diagram for a prior art bypass device 10 is shown in FIG. 1, in which a diode 22 is connected across the cell 20 such that the cathode of the diode 22 is connected to the positive terminal of the cell 20, and the anode of the diode 22 is connected to the negative terminal of the cell 20. If the voltage across the diode 22 is negative at the anode and positive at the cathode, as in normal operation of the cell 20, no significant current flows through the diode 22. If the cell 20 fails to an open-cell condition, for example, the voltage across the diode 22 reverses, and current flows through the diode 22 in the forward direction. Current flowing through the diode 22 causes the diode 22 to heat substantially, to at least 183 degrees C. A mass of a fusible material 24 is positioned at an initial mass location such that it is not within the shorting gap 30, but such that it is heated and melted by the heat produced by the diode 22. The melted fusible material 24 is driven into the shorting gap 30 and serves to cause the shorting gap 30 to be closed, which closure is indicated schematically by a switch 32 in FIG. 1.
The prior art bypass device circuitry shown in FIG. 1 has a significant shortcoming because if the single diode 22 suffers a short-circuit failure, the bypass device 10 could be inadvertently activated even when the cell 20 is not defective or in an open-cell condition. This could cause the cell 20 to overheat, vent, and cause a fire. Even apart from the use of a single diode 22, the prior art bypass circuitry is activated only when the heat from the diode 22 creates an effective thermal path to the fusible material 24, such that the fusible material 24 melts. If the heat dissipates as it travels along the thermal path from the diode 22 to the fusible material 24 such that it is then insufficient for heating the fusible material 24, the bypass device 10 fails. Finally, in the prior art bypass device 10, the fusible material 24 is unconstrained, which means that the molten flow is not guaranteed to close the shorting gap 30 for activation of the device, thereby resulting in device unreliability.
Thus, there is a need in the art for an improved technique for achieving an electrical bypass of electrical storage cells. The present invention fulfills that need, and further provides related advantages.